For many different types of storage media, the speed with which data may be retrieved varies depending upon where the data is stored on the storage medium. For example, with hard disk drives the retrieval value varies based on physical factors such as seek time, rotational delay, and disk transfer rate. Examples of other storage media for which retrieval value differs based on where the data is stored include, hybrid drives (storage media have rotating platters as well as a solid state component such as a flash component that can store data), and network attached storage such as iSCSI. The retrieval value may be affected by factors other than the physical characteristics of the storage medium, such as the communication link, or network (if used).
Many manufactures of storage media provide information that roughly describes how fast the storage media will provide data. For example, a manufacturer may provide an “average access time”. In practice, the term “average access time” is used to describe how fast a storage medium is. A storage medium with a lower average access time is generally more responsive and “peppier” than one with a higher average access time. But “average access time” is just an average for the whole storage medium. The results may vary for individual accesses to data stored in different locations on that same storage medium. Moreover, manufacturers may quote internal disk transfer rates (the rate that data can be written to or read from the disk) as a range of values. For example, a range might be a minimum of 43 Mbytes per second and a maximum of 78 Mbytes per second. However, the actual transfer rate at any particular place on the disk may be anywhere between the two extremes.
Thus, the actual speed with which data can be read from a storage medium is difficult or impossible to know based solely on manufacturer specifications. Moreover, factors external to the storage medium affect the rate at which data can be retrieved from a storage medium. For example, if a host computer system is connected to one storage medium via a network (“network storage medium”) and another directly via a SCSI cable (“SCSI storage medium”), the retrieval value for the network storage medium may on average be slower than the SCSI storage medium. However, some regions of the network storage medium may have a faster retrieval value than some regions of the SCSI storage medium.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.